readelf: Don't allocate string with asprintf, but reuse buffer with sprintf.

readelf: Don't allocate string with asprintf, but reuse buffer with sprintf.

Since we are single threaded we can just use a static result buffer for
format_dwarf_addr as long as we make sure to print the result before
calling format_dwarf_addr again. This removes lots of malloc/free calls.

On my machine eu-readelf -N --debug-dump=info libxul.so
goes from 57 seconds to 55 seconds.

Signed-off-by: Mark Wielaard <mark@klomp.org>